Electrolysis unit

ABSTRACT

An electrolysis unit having a plurality of membrane electrolytic cells connected in series suitable for use in a service station for supplying a motor vehicle with hydrogen as fuel. Each of the membrane electrolytic cells has a membrane provided on both sides with an appertaining contacting disk, especially a support mat made up of a large number of metal wires laid over each other and pressed together. In a service station, a gas cleaning unit is installed on the inlet side upstream from such an electrolysis unit.

BACKGROUND

[0001] The present invention relates to an electrolysis unit with anumber of membrane electrolytic cells electrically connected in series,each of which comprises a membrane surrounded by a pair of contactplates, whereby each contact plate has a channel system on its sidefacing the appertaining membrane for transporting water and/or gas. Thepresent invention also relates to a service station for supplying amotor vehicle with hydrogen as fuel.

[0002] In an electrolysis device, a medium is electrolyzed by applying asupply voltage between an anode and a cathode. When water is used as themedium, hydrogen and oxygen are formed in this process. Such anelectrolysis device can thus be used to generate hydrogen and/or oxygenas needed.

[0003] An electrolysis device can be configured as a so-called membraneelectrolyzer. In this case, the electrolysis device comprises a numberof membrane electrolytic cells in which the functioning principle is theopposite of that of a fuel cell as described, for example, in the Germanarticle titled “Brennstoffzellen für Elektrotraktion” [Fuel Cells forElectrotraction], by K. StraBer, VDI-Berichte [Reports of the GermanEngineering Association], No. 912 (1992), p. 125 ff. With such amembrane electrolytic cell, the water provided as the medium is conveyedto a membrane arranged between the anode and the cathode, especially toa cation exchanger membrane provided as the electrolyte. The membranehere is normally provided with a contact layer on both sides, wherebythe first contact layer serves as the anode and the second contact layerserves as the cathode. Such a membrane electrolytic cell stands out forits especially compact design so that an electrolysis unit with a numberof membrane electrolytic cells can be accommodated in a very smallspace. In order to feed the requisite media, especially the water to beelectrolyzed, and in order to discharge the generated gas, the membranein such a membrane electrolytic cell is normally provided on both sideswith an appertaining contact plate that has a channel system fortransporting the medium on its side facing the membrane.

[0004] The hydrogen that can be generated as needed in such anelectrolysis unit is especially useful as a mobile fuel that couldconstitute an environmentally sound alternative to the widespread use offossil fuels. In particular, the generated hydrogen, if necessary at anappropriate temperature in liquefied form, is especially suitable as amobile and flexibly employable fuel for an environmentally sound use inmotor vehicles of all kinds. Particularly with an eye towards efforts toreduce the global consumption of fossil fuels in supplying motorvehicles with power, a greater use of hydrogen as fuel for motorvehicles would be desirable.

[0005] In order for hydrogen to become more widespread as fuel for theoperation of motor vehicles, however, there is a need to establish asufficiently dense network of service stations in which the necessaryhydrogen can be supplied in a simple and safe manner. For this purpose,flexible installations are needed in which the hydrogen can be generatedat an adequate capacity and at an adequate rate while adhering to a highlevel of safety. A membrane electrolyzer is fundamentally well-suitedfor this purpose, although so far, the required capacities andproduction rates have only been achieved to an insufficient extent.

SUMMARY OF THE INVENTION

[0006] The present invention provides an electrolysis unit of the typedescribed above with which a high production capacity of hydrogen can beachieved with relatively few resources and with simple means. Moreover,the present invention provides a service station for providing areliable supply of hydrogen as fuel for a motor vehicle.

[0007] The electrolysis includes a contacting disk arranged between eachcontact plate and the appertaining membrane.

[0008] The production capacity of a membrane electrolyzer depends, amongother things, on the total current that can be applied to the entiresystem in order to electrolyze the water to form hydrogen and oxygen.For material-related reasons, especially in view of the materialproperties of the membrane that is used in the membrane electrolyticcells, however, the current density in the individual membraneelectrolytic cells is limited. Therefore, an increase in the totalproduction capacities that can be achieved is attainable by increasingthe active surface area provided per membrane electrolytic cell.However, increasing this surface area means that the reliable electriccontacting of the sides of the membrane over a large surface area couldbecome problematic, especially if the membrane used becomes unstablebecause the selected surface area was too large and then tends to becomedeformed due to the relatively high mechanical loads. Consequently, inorder to ensure a reliable electrical contact over a large surface areabetween the contact plate and the membrane even when a large totalsurface area is selected, a contacting disk is provided between thecontact plate and the membrane, whereby said contacting disk has a highelectrical conductivity on the one hand and a high permeability to themedium on the other hand.

[0009] In order to also keep the mechanical stress on each membrane lowin the case of such improved contacting, the mechanical properties ofthe contacting disk should be systematically configured so as toadditionally support the membrane. Here, this support should be designedin such a way that the current flow in the entire system is minimallyimpeded. These criteria are met with a support that concurrently actsover a large surface area in that each contacting disk advantageouslyhas a high conductivity and also a high porosity of up to about 50%.

[0010] With an eye towards the above-mentioned criteria, the contactingdisk could be configured, for example, as a metal contact piece,especially made of titanium, which is provided with a large number ofmicrofine bores having a diameter, for instance, of 100 μm. As analternative, the contacting disk could also be in the form of a metallicsintered element that, in a sponge-like configuration, has a largenumber of cavities for whose formation, for example, suitable plasticcan be used that is volatile at elevated temperatures. Advantageously,however, the contacting disk in question is configured as a support matwhich, owing to its mechanical properties, especially a certainplasticity or deformability, ensures a particularly intense electricalcontact over a large surface area when the membrane is affixed betweenthe contact plates.

[0011] With such an arrangement, in order to ensure a sufficiently highconductivity of the entire system on the one hand and a sufficientlyhigh porosity of the support vis-à-vis the medium flows on the otherhand, the support mat or each support mat is advantageously made up of alarge number of metal wires laid over each other and pressed together.The support mat, which thus has a felt-like appearance, is preferablymade of titanium wires, in order to ensure sufficient resistance and arelatively long service life, even under conditions that are aggressiveto the material. Particularly when titanium wires are used, asufficiently high conductivity and corrosion resistance of the entiresystem can be achieved.

[0012] A high flexibility even when different numbers of membraneelectrolytic cells are combined to form the electrolysis unit can alsobe achieved with a compact design in that the channel system of thecontact plate or of each contact plate is advantageously connected to afeed or discharge channel for the medium by means of a shut-off valvethat is integrated into each contact plate in such a way that it can beshut off. In particular, there can be a shared feed or discharge channelfor the medium for all the contact plates, said channel being formed ineach contact plate by an interconnected opening in the plane of theplate. When the contact plates are combined in order to form theelectrolysis unit, in which process the contact plates are stacked ontop of each other, these openings come to lie above each other, thusforming a medium channel that extends in the stacking direction of thecontact plates. With this embodiment, the appertaining channel system ofthe contact plate can be connected to the appertaining media channel viaa branch channel that extends in the plane of the plate, whereby theshut-off valve opens or closes the appropriate branch channel as neededvia a suitable actuation from the outside.

[0013] In a flexible manner and with a relatively high productioncapacity, the electrolysis unit allows a decentralized production ofhydrogen by electrolysis of water and it is thus well-suited forestablishing a widespread supply network for motor vehicles. Therefore,in an advantageous embodiment, the electrolysis unit is part of aservice station used for supplying motor vehicles.

[0014] The service station according to the present invention includes apumping system connected on the inlet side via a gas cleaning unit to anelectrolysis unit of the type described.

[0015] Here, it is firstly taken into account that the electrolysis unitof the type described should indeed ensure an adequately high productionrate of hydrogen while having a relatively compact design, which isnecessary for the reliable operation of a widespread supply network.Moreover, it is also taken into account that, in modern, hydrogen-baseddrive concepts for motor vehicles, as we move away from conventionalcombustion engine technology, more and more fuel cell-based conceptswill be used. In such fuel cells, in a kind of reversal of the describedelectrolyzer concept, a systematic recombination of hydrogen with oxygenwill be carried out making use of a suitable membrane or separatinglayer, a process in which electric power is liberated. This, in turn,can be used to power the motor vehicle. For the operation of suchsystems, which generally stand out for their relatively high efficiencyand operational reliability, however, a high level of purity of about99.99% is desirable for the supplied hydrogen. In order to supplyhydrogen of such a high purity in an adequate quantity in the servicestation, the electrolysis unit of the type described above is combinedwith an appertaining gas cleaning unit. In this context, the gascleaning unit is specially configured for removing oxygen from the gasflow.

[0016] In order to achieve a high operational reliability of the fillingstation and particularly of the electrolysis unit used therein, thecontacting disk of one or of each membrane of the electrolysis unit ispreferably electrically connected to an analyzer that determines thedecay time of a voltage signal at this membrane when the current supplyto a membrane is switched off. This is based on the insight that anoperational failure of a membrane electrolytic cell can frequently betraced back to damage to its membrane, for example, as a result of holeformation. Such damage to a membrane due to hole formation can bedetected especially easily by measuring at the membrane the course overtime of the voltage that drops after the current supply has beenswitched off. This is because, in such a case, the membrane electrolyticcell to be examined should briefly behave like a fuel cell since thereare still residues of the previously generated hydrogen or oxygen onboth sides of the membrane. Therefore, if the membrane is intact, thevoltage that drops at the membrane should briefly remain constant beforethe voltage signal decays. In contrast, if the membrane is damaged, thedecay of the voltage signal sets in relatively sooner. Consequently, bydetermining the decay time of the voltage signal, a conclusion can bedrawn about the condition of the membrane. Thus, a defective membraneelectrolytic cell can be identified in a particularly simple manner.

[0017] In order to attain a high production rate of the electrolysisunit on the one hand, and a long service life of the electrolysis uniton the other hand, along with correspondingly little maintenance work,the membrane of each membrane electrolytic cell advantageously has acontact layer made of platinum as the cathode and a contact layer madeof iridium as the anode.

[0018] In order to ensure a high quality of the hydrogen supplied by theservice station, in an advantageous embodiment, the gas cleaning unithas a water separator and a dryer system connected in series with thelatter on the gas flow side. Thus, any entrained residual water thatmight still be present in the generated gas flow can be separated fromthe gas flow. These dryers are advantageously configured forregeneration by back-washing, so that the operational and maintenancework can be kept especially simple, even in the case of “spent” activedryer components. For this purpose, in another advantageous embodiment,the dryer system comprises at least two dryers connected in parallel onthe gas flow side, whereby each dryer is installed in a main gas linegroup as well as in a branch line group through which the flow can movein the opposite direction from that in the main gas line group.

[0019] The advantages achieved with the present invention consistespecially in that, through the use of the contacting disks, especiallythe support mats, the requirements for a reliable generation of evenrelatively large quantities of hydrogen can be met in an advantageousmanner. On the one hand, the contacting disks, due to their highporosity, allow exposure of each membrane over a large surface area tothe necessary media, whereby on the other hand, with reliable electricalcontacting over a large surface area, a reliable mechanical support ofthe membranes can be effectuated. This allows an enlargement of theactive membrane surface area to a great extent so that, as a result ofthe available, relatively large active surface area, a correspondinglyhigh production rate of hydrogen can be achieved. Therefore, through thecombination of such an electrolysis unit with the other components, suchas especially the gas cleaning installation, a service station can beprovided which, with the relatively high specifications, allows areliable and widespread network that can supply hydrogen as a possiblemobile fuel for motor vehicles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] An embodiment of the present invention will be explained ingreater depth with reference to the drawings, in which:

[0021]FIG. 1 shows a service station for a motor vehicle;

[0022]FIG. 2 shows a gas cleaning unit of the service station accordingto FIG. 1;

[0023]FIG. 3 shows an electrolysis unit in a longitudinal section;

[0024]FIG. 4 shows a section from FIG. 3 in an enlarged view;

[0025]FIG. 5 shows the electrolysis unit according to FIG. 3 in a crosssection;

[0026]FIG. 6 shows a shut-off valve in a longitudinal section; and

[0027]FIGS. 7a, 7 b show the shut-off valve according to FIG. 6 in aclosed and open position.

[0028] The same parts are marked with the same reference numerals in allof the Figures.

DETAILED DESCRIPTION

[0029] The service station 1 according to FIG. 1 serves to supply amotor vehicle 2 with hydrogen as fuel. For this purpose, there is astorage and filling system 4 that comprises a pumping system 6 fordispensing the fuel. On the fuel side, on the inlet side, the pumpingsystem 6 is connected to a number of storage tanks 8 in which thehydrogen intended to be dispensed as the fuel is kept at a high pressurethat is suitable for storage and dispensing.

[0030] The storage system 8, in turn—via a feed line 10 in which acompressor station 12 that is suitable for establishing the highpressure needed in the storage tanks 8 is installed—is connected on theinlet side to a system 14 that serves to generate the hydrogen intendedas the fuel.

[0031] The system 14 is configured for generating hydrogen as needed bymeans of the electrolysis of water to form hydrogen and oxygen. For thispurpose, the system 14 comprises a water circulation system 16 in whichan electrolysis unit 20 is installed. Here, the electrolysis unit 20 issupplied on the inlet side with water at a suitably selected operatingpressure of, for example, 5 to 30 bar, by pumps 22 installed in thewater circulation system 1. The electrolysis unit 20 comprises a cathode24 and an anode 26, which, during operation, are supplied with asuitably selected control voltage. Through the electrolysis of part ofthe water being fed in, hydrogen is generated at the cathode 24 andoxygen is generated at the anode 26.

[0032] The hydrogen generated at the cathode 24, together with theremaining water, is fed via a drain 28 to a gas/water separator 30. Inthe gas/water separator 30, which is connected on the gas side with thefeed line 10, the entrained hydrogen is separated from the water.Analogously, the anode 26 is connected via a drain 32 to a gas/waterseparator 34 in which the entrained oxygen is removed from the waterflow. On the gas side, the gas/water separator 34 is connected via adrain 36 to a drain system 38 for oxygen. On the water side, the waterseparators 30, 34 are connected by combined water lines to the inletside of the pumps 22 so as to form a closed water circulation system 16.As an alternative, two independent water circulation systems could alsobe provided on the one hand for the cathode side and on the other handfor the anode side. In order to compensate as needed for the water thathas been electrolyzed in the electrolysis unit 20, a water supply linegroup 40 is connected to the water circulation system 16, and a watertreatment unit 42 as well as a supply pump 44 are installed in saidwater supply line group 40. In order to establish adequate operatingconditions in the water circulation system 16, it also contains a numberof additional functional components such as, for example, a heatexchanger 46, a catalyst arrangement 48 (not necessary with two separatewater circulation systems) or a buffer tank 50.

[0033] In order to avoid the formation of explosive mixtures in the feedline 10 before starting up or after switching off the system 14, a purgegas line 51 is connected to the feed line 10. Via this purge gas line51, a purge gas, especially nitrogen, can be fed into the feed line 10if needed.

[0034] In the embodiment, the motor vehicle 2 is fitted for operationwith fuel cells 52 that can be supplied with fuel via a hydrogen tank54. Particularly when fuel cells are used for energy conversion,however, special requirements exist in terms of the quality and purityof the supplied hydrogen. In order to reliably comply with theserequirements, the service station 1 has a gas cleaning unit 60 on theinlet side in the feed line 10 upstream from the pumping system 6. Thegas cleaning unit 60 ensures that the hydrogen provided by the pumpingsystem 6 has a purity, for instance, of 99.99%.

[0035] For this purpose, the gas cleaning system 60 is designed so as toconsistently remove oxygen residues or water fractions that might havebeen entrained in the hydrogen flow. In order to achieve this, the gascleaning unit 60 installed in the feed line 10, as schematically shownin FIG. 2, comprises a catalytic recombiner 62 in which, by means of acatalyst located there, for example, on the basis of palladium, anyoxygen that might have been entrained in the gas flow is systematicallyrecombined with hydrogen to form water. Downstream from the recombiner62, there is a condenser 64 in which the gas flow that was heated up dueto the exothermal reaction in the recombiner 62 is cooled off again. Asa result of this cooling off, any moisture that might have beenentrained then condenses out.

[0036] The condensed-out moisture is subsequently separated in acondensate trap 66 located downstream from the condenser 64.

[0037] In order to further dry the gas flow, downstream from thecondensate trap 66, there is a drying system 68 that comprisesessentially two dryers 70, 72 connected in parallel on the gas flowside. On the outlet side, the dryers 70, 72 are connected to the feedline 10 via appertaining outflow lines 74, 76 on the outlet side.

[0038] In addition or as an alternative, there could also be a catalyticrecombiner installed in the drain 28 and said catalytic recombiner couldbe coated with catalytically active material such as, for example, anoble metal, in the area where its surfaces come into contact with thegas flow. This recombiner can already bring about a recombination of anyoxygen that might still be entrained with hydrogen in a given gas flow,whereby the water generated in this process is also separated in thegas/water separator 30 that is downstream from this anyway. In thiscase, a reliable removal of possibly entrained oxygen can be effectuatedwith especially simple means. If necessary, the recombiner, which canhave, for example, a space filled with packings having catalyticallyactive surfaces as a catalytically active zone, can also be combinedwith a static mixer for thorough mixing of the gas fractions containinghydrogen and oxygen with each other.

[0039] The gas cleaning unit 60 is designed so as to allow aregeneration of the dryers 70, 72 by means of back-flushing as needed.For this purpose, each dryer 70, 72 has an appertaining back-flushingline 78, 80. Therefore, in conjunction with suitable branch lines 82, 84and a back-flushing line 86, the dryer system 68 can be operated in sucha way that one of the dryers 70, 72 is operated in the main flowdirection, whereby regeneration gas R flows through the other dryer 72or 70 in the opposite direction, regenerating it in the process. Thus,the affected dryer 70 or 72 can be regenerated without any interruptionin the operation. Naturally, functional switch-over or shut-off valvesare mounted at appropriate places in the above-mentioned lines.

[0040] All in all, the service station 1 is designed with an eye towardsestablishing a widespread supply network to supply vehicles 2 withhydrogen as fuel. For this purpose, especially the production capacityof the service station 1 to provide the requisite hydrogen isdimensioned accordingly high. In order to render this possible, theelectrolysis unit 20 is specifically configured for a high productioncapacity for hydrogen while concurrently having a particularly compactdesign.

[0041] For this purpose, the electrolysis unit 20, as is shown in thelongitudinal section in FIG. 3, is configured as a membrane electrolyzerand comprises a number of membrane electrolytic cells 90 electricallyconnected in series. In the embodiment according to FIG. 3, fivemembrane electrolytic cells 90 connected in series are shown; however,any desired number of membrane electrolytic cells 90 can be provided. Ascan be seen in the enlargement in FIG. 4, each membrane electrolyticcell 90 has a membrane 92 configured in the form of a cation exchangermembrane as an electrolyte for water as the medium to be electrolyzed.The membrane 92 of each membrane electrolytic cell 90, which canespecially be configured as a fiber-reinforced PEM membrane of the typeavailable under the description “Nation 424”, is provided on both sideswith a contact layer (not shown here). The two contact layers of amembrane 92 serve as electrodes in the electrolysis procedure. In theembodiment, the contact layer of each membrane 92 provided as thecathode is made of platinum. In contrast, the contact layer of eachmembrane 92 provided as the anode is made primarily of iridium.

[0042] On each contact layer of each membrane 92, there is a contactplate 94. The supply of each membrane 92 with the electrolytic currentneeded for the electrolysis of the water on the one hand and with themedia needed there on the other hand, especially water, is effectuatedvia the contact plate 94. The membrane electrolytic cells 90 consistingin each case of a membrane 92 and the appertaining contact plates 94 arein a stacked arrangement. Adjacent contact plates 94 can be electricallyseparated from each other by an insulator plate (not shown here),whereby the series connection of the membrane electrolytic cells 90 iseffectuated by an external line system (not shown here). As analternative, adjacent contact plates 94 of various membrane electrolyticcells 90 can also be directly in electrical contact with each other orcan also be constructed in one piece. On the faces 100 of the stackformed by the membrane electrolytic cells 90, there are a number ofscrews that serve as fastening elements 102 for affixing the membraneelectrolytic cells 90 to each other. In addition to that, the inside ofeach membrane electrolytic cell 90 can have force sensors, for example,strain gauges, that allow monitoring of the forces exerted on theindividual components when the fastening elements 102 are affixed. Inthis manner, an overloading of the components can be avoided.

[0043] In order to ensure a high production rate of hydrogen, theelectrolysis unit 20 is designed, on the one hand, for a high currentdensity, particularly on the basis of the material properties of themembranes 92 and, on the other hand, for a generously dimensioned activesurface of the membranes 92 with a diameter, for example, of 390 mm.Particularly in view of such a large dimensioning and of the relativelyhigh operating pressures of about 5 to 30 bar that are conceivableduring the operation of the electrolysis unit 20, the membranes 92,however, are exposed to a relatively high mechanical load duringoperation. In order to ensure a reliable electric contact of themembranes 92 over a large surface area under these conditions as well,and thus to keep the operational reliability of the electrolysis unit 20especially high, each membrane 92 is provided with an appertainingcontacting disk 104 on both sides, as can be seen in FIG. 4. On thecathode side of the membrane 92, on which the formation of the hydrogengas can be expected, there is also a buffer element 106 between themembrane 92 and the contacting disk 104; in the embodiment, said bufferelement 106 is made of carbonized paper that is about 0.35 mm thick. Ineach case, the contacting disk 104 is selected in such a way that on theone hand, it ensures a greater electric contact in the current flowdirection, that is to say, from each contact plate 94 to theappertaining membrane 92. On the other hand, however, the contactingdisk 104 is also selected in such a way that it greatly promotes themedia flow from each contact plate 94 to the appertaining membrane 92.Media flow here refers, on the one hand, to the supply of water to theappertaining membrane 92 and, on the other hand, to the removal of thegases generated during the electrolysis, that is to say, especiallyhydrogen and oxygen. The buffer element 106, i.e. the carbonized paperin the embodiment, is somewhat deformable or compressible under pressureso that, even if there are uneven spots, a uniform contact over a largesurface area is ensured.

[0044] In order to fulfill the above-mentioned requirements, thecontacting disk 104 has a high electrical conductivity as well as a highporosity of up to about 50%. For this purpose, the contacting disk 104in the embodiment is configured as a support mat, whereby each supportmat is like a felted fabric consisting of a large number of titaniumwires laid over each other and pressed together. As an alternative,however, the contacting disk 104 could also be in the form of a suitablemetal piece, especially of titanium, that is provided with a largenumber of fine boreholes. As a result, it is especially ensured that thecontacting disk 104 can be configured so as to be plane-parallel, thuspromoting a flat and continuous, high-quality contact between eachcontact plate 94 and the appertaining membrane 92. In the embodiment asa support mat, the resultant elastic deformability makes it possiblethat, as a result of the fixation with the fastening screws 102, a closecontact exists over a large surface area between each contact plate 94and the appertaining membrane 92.

[0045] Each contact plate 94, as shown in FIG. 5 with reference to theelectrolysis device 20 depicted in a cross sectional view, is designedso as to be virtually circular and it has a channel system 110 on itssurface facing the appertaining contact layer. The channel system 110 ismade up of indentations extending into each contact plate 94 and theyare arranged in the form of concentric circle segments on the surface ofeach contact plate 94. The channel system 110 of each contact plate 94serves to transport the medium to be electrolyzed to the appertainingmembrane 92. For this purpose, the channel system 110 of each contactplate 94 is connected to a feed system for an electrolytic medium.Moreover, a discharge system for gas or for gas mixed with electrolyticmedium is connected to the channel system 110 of each contact plate 94.

[0046] In order to form a shared feed and discharge system for therequisite electrolytic medium and/or for the gases generated during theelectrolysis, the contact plates 94 are provided with a number ofcontinuous passage openings 112 which completely penetrate the baseplane of the appertaining contact plate 94, whereby each opening can besealed vis-à-vis the environment by means of an encircling groove 114with an O-ring. The openings 112 are arranged in such a way that, whenseveral contact plates 94 are stacked one after the other to form theelectrolysis unit 20, the corresponding openings 112 of each contactplate 94 come to lie behind each other, thus forming in their entirety achannel through which the media can flow in the lengthwise or stackingdirection of the electrolysis unit 20.

[0047] In this manner, the openings 112 that are associated with eachother form a number of shared feed or discharge channels for media forthe entire electrolysis unit 20.

[0048] In order to be able to switch individual membrane electrolyticcells 90 on or off as needed, and thus, for example, to deactivatemembrane electrolytic cells 90 that have become defective without havingto switch off the entire system, the channel system 110 of each contactplate 94 is connected to the shared feed or discharge channels for mediaso that the former can be individually shut off. For this purpose, anumber of shut-off valves 120 are integrated into each contact plate 94,whereby the integrated construction is selected in such a way that theoverall height or the total thickness of the individual contact plates94 does not change as a result of the shut-off valves 120. The interiorof each opening 112 associated with the feed or discharge channel forthe media is connected to the channel system 110 of each contact plate94 via a branch channel 122, whereby the shut-off valve 120 opens orcloses the appertaining branch channel 122 as needed.

[0049] An embodiment for the shut-off valve 120 is shown in FIG. 6. Theshut-off valve 120 comprises a valve finger 123 that can be moved in itslengthwise direction and that opens up at the end into a widened valvehead 124. The valve head 124 corresponds to a valve seat 122 on the endof the appertaining branch channel 126 on the outlet side. The valvefinger 123 can be moved in its lengthwise direction by means of acontrol mechanism 128 that can be actuated, for example, electrically,hydraulically or electro-hydraulically. As can be seen in FIG. 6, all ofthe components of the shut-off valve 120 are dimensioned so as not toexceed the total prescribed thickness of the contact plate 94. Insteadof the shut-off valve 120, a screw could be provided that can be appliedwith its thread onto the valve seat 126.

[0050] In FIG. 7, the shut-off valve 120 is shown in the closed (FIG.7a) as well as in the open (FIG. 7b) position. As can be seen in thedepiction of FIG. 7a, when the shut-off valve 120 is in the closedposition, the valve head 124 is positioned in the appertaining valveseat 126, thus blocking the appertaining branch channel 122.Nevertheless, a cross section albeit reduced—remains free in the opening112 for the appertaining medium to flow through, so that a systematicswitching off of the appertaining branch channel 122 is possible withoutappreciably influencing the media flow in the actual feed or dischargesystem. In the open position (FIG. 7b), however, the valve finger 123 iscompletely retracted so that the medium can freely access theappertaining branch channel 122.

[0051] Therefore, by using the shut-off valves 120 on the medium flowside, individual membrane electrolytic cells 90 can be switched on andoff relative to the overall system. As a result, individual membraneelectrolytic cells 90 can be selectively checked for proper functioning,for example, by exposing them to a medium under excess pressure. If amembrane electrolytic cell 90 is recognized as being defective in thisprocess, then this cell can be bypassed on the flow side as well as onthe medium side.

[0052] In the service station 1 shown in FIG. 1, the electrolysis device20 has an appertaining analyzer 130 which, as indicated by the arrows132, is electrically connected to the contact layers of each membrane92. For this purpose, after the power supply has been switched off, theanalyzer 130 is designed so that it can determine the decay time of avoltage signal at this membrane 92. Then, on the basis of the decay timeof the voltage signal, the analyzer 130, can provide information aboutthe proper functioning of each membrane 92. Thus, if the membrane 92 isintact, then the appertaining membrane electrolytic cell 90 should stillact as a fuel cell for a brief time after the power supply has beenswitched off, until the gases it had previously released have beentransported away. Therefore, if the membrane 92 is intact, then thedecaying voltage signal should at first be constant for a brief timebefore a decay sets in. In contrast, if the membrane 92 is defective,for example, because of hole formation, then the voltage should decayimmediately after the power supply has been switched off, and in thismanner, the analyzer 130 can distinguish an intact membrane 92 from adefective one.

What is claimed is:
 1. An electrolysis unit including a plurality ofmembrane electrolytic cells electrically connected in series, each ofthe plurality of cells comprising: a membrane; two contact plates eachdisposed on an opposing side of the membrane and each including achannel system configured to transport at least one of a water and a gason a side of the contact plate facing the membrane; two contactingdisks, each disposed between one of the two contact plates and themembrane.
 2. The electrolysis unit as recited in claim 1, wherein eachof the two contacting disks includes a support mat.
 3. The electrolysisunit as recited in claim 1-, wherein each support mat includes aplurality of metal wires laid over each other and pressed together. 4.The electrolysis unit as recited in claim 2, wherein at least onesupporting mat includes titanium.
 5. The electrolysis unit as recited inclaim 1, wherein each channel system includes a connection to asupporting channel and wherein each contact plate includes a shut-offvalve for shutting off the connection.
 6. The electrolysis unit asrecited in claim 5, wherein the supporting channel includes one of afeed channel and a discharge channel.
 7. A service station for supplyinga motor vehicle with hydrogen as fuel comprising: a pumping systemhaving an inlet; a gas cleaning unit; and an electrolysis unit connectedto the pumping unit at the inlet via the gas cleaning unit, wherein theelectrolysis unit includes a plurality of membrane electrolytic cellselectrically connected in series to each other, and wherein each of thecells includes a membrane, two contact plates, and two contacting disks,wherein each of the contacting plates is disposed on an opposing side ofthe membrane and includes a channel system configured to transport atleast one of a water and a gas on a side of the contact plate facing themembrane, and wherein each contacting disk is disposed between one ofthe two contact plates and the membrane.
 8. The service station asrecited in claim 7, further comprising an analyzer electricallyconnected to at least one of the contacting disks of the electrolysisunit, the analyzer configured to determine a decay time of a voltagesignal at the membrane when a power supply to the membrane is switchedoff.
 9. The service station as recited in claim 7 wherein one of the twocontact layers of each membrane is a cathode and includes platinum andthe other of the two contact layers of each membrane is an anode andincludes iridium.
 10. The service station as recited in claim 7 whereinthe gas cleaning unit includes a water separator and a dryer systemconnected in series on a gas flow side.
 11. The service station asrecited in claim 10, wherein the dryer system includes at least twodryers connected in parallel on an inlet side of the cleaning unit. 12.The service station as recited in claim 11 wherein each of the at leasttwo dryers is installed in a main gas line group and in a branch linegroup, a flow of gas moving through the branch line group in an oppositedirection as through the main gas line group.